Antibodies against double stranded (ds) DNA are a characteristic serologic hallmark of SLE. While it has been demonstrated that anti-dsDNA antibodies play a critical role in the pathogenesis of lupus nephritis, the mechanisms of injury are incompletely understood. Experimental evidence strongly suggests that at least some anti-dsDNA antibodies are pathogenic by virtue of their direct cross-reactivity with renal antigen. We recently demonstrated that a pathogenic anti-dsDNA antibody (R4A) binds to a 100 kD protein expressed on the cell surface of a mesangial cell line derived from a lupus-prone MRL-lpr/lpr mouse, and that DNAse treatment of the lysate does not affect binding. Binding was greatly diminished in lysates of a mesangial cell line derived from a non-autoimmune mouse, suggesting that antigen expression and/or availability at the level of the target organ may be a factor in determining susceptibility to lupus nephritis. Following identification of the 100 kD protein bound by R4A as alpha-actinin, the binding of R4A to alpha-actinin was confirmed by Western blot, ELISA, and inhibition studies. High titers of anti-alpha-actinin antibodies were found in the serum and kidney eluates of lupus mice with nephritis, and in the serum of lupus patients. The goals of this proposal are to study if cross-reactivity with alpha-actinin may be an important determinant of the renal pathogenicity of some anti-DNA antibodies, and if the expression of alpha-actinin is genetically regulated and modulated by gender and exposure to cytokines. We will determine if anti-alpha-actinin antibodies are pathogenic and if they cross-react with dsDNA, by immunization of mice with alpha-actinin and studying the anti-alpha-actinin antibody response in normal and autoimmune mice. The molecular basis for the differential levels of antigen display of alpha-actinin between autoimmune and non-autoimmune mouse strains will be studied, and the effects of age, gender, and cytokines known to be present in lupus kidneys on alpha-actinin expression and antibody binding will be determined. Finally, we will identify the epitopes of alpha-actinin that are recognized by pathogenic anti-dsDNA antibodies to understand the generation of anti-alpha-actinin antibodies, and determine the potential of alpha-actinin peptides in the treatment of acute lupus nephritis.